Electronic integrated circuitry is often tested under simulated normal operating conditions after manufacture in order to identify circuit defects. The circuitry may be subjected to more stressful conditions, such as operating the circuit in a heated environment, in order to exaggerate the failure rate of defective circuitry while ensuring greater reliability of the circuitry which survives the environmentally stressful conditions. Because the rate of electronic component failure is significantly higher during the early life of a component (the so-called infant mortality rate), testing under environmentally stressful conditions is an effective method of identifying integrated circuits which would likely experience early failure in the field.
In recent years, electronic integrated circuits have become increasingly smaller and more densely packed onto circuit cards or carrier modules. This increased density of circuits has limited the space available on the cards or modules for circuit interconnection elements. The advent of flip-chip technology has partially alleviated the space problem by permitting designers to eliminate pin packages for the integrated circuits and the associated pin interconnection elements. Using flip-chip technology, an integrated circuit chip cut from a wafer may be directly attached to interconnection points such as solder pads on the card or module, by means of a solder reflow process which forms an electrically conductive junction between the integrated circuit chip and circuit lines on the card or module.
Environmental testing of integrated circuits used in flip-chip technology presents a problem not encountered with testing integrated circuits encapsulated in pin packages. Pin packaged integrated circuits may be tested while being temporarily mounted onto test cards having sockets for the pins. Temporary mounting of integrated circuits used in flip-chip processes, however, is not an acceptable alternative. Temporary mounting requires subjecting the integrated circuit chip to a first solder reflow process to attach the integrated circuit to the carrier prior to testing, and a second solder reflow process to detach the chip upon completion of the test. This second solder reflow process is particularly undesirable in that it subjects the integrated circuit chip to heat and excessive handling after the testing has been completed. Any damage caused to the chip during removal of the chip from the carrier will be undetectable, resulting in defective integrated circuits which are determined to be acceptable.
Thus there is a need to be able to test integrated circuits used in flip-chip technology without temporarily mounting the circuit chips on a carrier module. This need is further demonstrated by the fact that circuit card or carrier module manufacturers often purchase integrated circuit chips to be used on the cards or modules from outside sources, thereby precluding any possibility of testing the integrated circuits while mounted on their intended cards or modules.
Due to the miniaturization of integrated circuit technology, handling and testing of individual integrated circuit chips is difficult. Environmental testing is thus best accomplished while the individual circuits reside on their respective wafers. Such a method of testing, however, presents its own problem due to spatial limitations. Each of the integrated circuits on the wafer has contact points on the surface of the wafer which must be contacted by a particular element of a multi-element test probe. The diminutive size of the test contact points requires that the elements of the test probe be of an appropriately small size, and further that the test probe elements be precisely aligned with the test contact points on the integrated circuits undergoing the test.
The precise alignment between the elements of the test probe and the test contact points on the integrated circuits is difficult to maintain over the temperature range utilized during environmental testing (e.g. burn-in testing). For example, heating the wafer, which is comprised of silicon, results in a minor expansion of the wafer due to the thermal expansion coefficient of silicon. Even this minor expansion, however, can cause problems due to the diminutive size of the test contact points on the integrated circuits on the wafer. As a result, the elements of the test probe may become misaligned with respect to these test contact points to such an extent that the probe elements no longer contact the test contact points. Maintaining alignment is of critical importance in most aspects of testing, e.g. preventing self-healing of some types of integrated circuit faults during cool down, maintaining contact during temperature step stress, maintaining contact during power step stress, or any changes in temperature or power with electrical contact required.
The problem may be solved if the multi-element probe is selected to possess the same or similar thermal expansion coefficient as that of silicon. Because it is desirable to construct the probe elements from a conductive metal, other portions of the test probe assembly must be selected so that, generally, the thermal expansion coefficient of the test probe assembly matches that of the silicon wafer. At the same time, the test probe assembly must provide means to effectively and accurately position the probe elements so that proper initial contact may be made with the test contact points on the integrated circuits being tested. The present invention provides a test apparatus having such a probe assembly, as well as a method of constructing the probe assembly.